In this case it is just ‘under’ . The text explains what a Raclette is -
“....c’est un gros fromage sous un fil chauffant que l’on racle ....“
It is an enormous cheese wheel placed UNDER a heated element and you scrape the melted cheese into your plate, normally served with potatoes, charcuterie, and gherkins ...Delicious!
In a restaurant it is quite a spectacle but you can now buy a home raclette which you just plug in and is much easier to use..
Hope this helps!